ANTI-RADAR NETS

Abstract: Anti-radar nets consisting of a fabric and a coating layer on both sides of the fabric, in which the weft threads of the fabric are provided with metal fibres and at least one of the coating layers is provided with carbon fibers, and process for manufacturing such anti-radar nets.

Description: "Anti-radar nets." The present invention relates to anti-radar nets. Anti-radar nets are known since many years. In general those nets consist of several layers of materials which partially absorb and partially reflect radar signals, at least one layer of which is a heterogeneous electricity conducting film which reflects part of the arriving radar waves. The reflected image should correspond to the surroundings where the concerned anti-radar nets are located. For that purpose, among other approaches there have been incorporated metal fibres into the nets, sometimes also a creased metal foil. All these materials require quite complex manufacturing processes which are rather expensive, among which the gluing of metal foil on textiles, without achieving however a satisfying attenuation of the frequencies above 30 GHz . The purpose of the present invention is to provide anti- radar nets with a high attenuation, above 10 dB, at high frequency waves ( from 8 to 94 GHz ) , measured according to the known method of a monostatic setting, in which a radar wave from a source is emitted through a chamber or tunnel which is completely covered with absorbing material, except one plate at the extremety of said tunnel, situated in the so called window, by which the radar wave is reflected to the source of which the antenna is now used as receiver. In order to test a radar net a piece of the material is provided between de source/receiver and the reference plate, and the reflected signal is compared with the reflected signal without the radar net. The ratio of these two signals is called attenuation and is expressed in dB. The anti-radar nets according to the invention consist of a fabric and a coating layer on both sides, characterised in that the weft threads of the fabric are provided with metal fibers and at least one of the coating layers is provided with carbon fibers. The invention also relates to a manufacturing method for anti-radar nets. The first anti-radar panels were obtained from expanded metal grids. After that one has adopted various foils with dispersed metal fibers mixed therein, and/or with a punched metal foil between a support fabric and an external coating layer of synthetic material. The manufacturing method of the anti-radar nets of the invention is characterised in that the support fabric comprises weft threads with metal fibres whereas the coating layers, eg. made of synthetic material, are provided with carbon fibres which are distributed and mixed through the coating layer matrix by a foaming technique and then applied as coating layer onto the support fabric, and then calendered. Further features and characteristics of the anti-radar nets according to the invention, and of the method for manufacturing those nets will become apparent from the following description. In order to manufacture an anti-radar camouflaging net, first a fabric is woven, the warp threads of which are preferably spun exclusively from so called High Tenacity polyamide ( HT pa ) and the weft threads of which consist partly of HT pa and partly of mixed yarn of polyamide - metal, in which the metal consists of rust-resistant fibers. In a typical embodiment of the fabric the warp is made of 235 dtex HT polyamide, at 11,7 fibres/cm - whereas for the weft one uses one shot of 235 dtex HT polyamide at 8 fibres/cm, followed by two shots of mixed yarn of 20 dtex polyamide with rust-resistant fibres in a ratio of 94 % polyamide and 6 % rust-resistant fibres, at 17 fibres/cm. The content of metal can range from 0,5 to 10 g/m2. In the preferred embodiment of the invention the fabric contains approximately 2 g of metal per m2. The composition of the fabric depends however to a large extent of the requirements set, eg. by user specifications, in respect of weight, tensile strenghth, tear resistance, etc... The use of metal in the weft allows to maintain a maximal tensile strength in the warp as well as a maximal processability of the fabric in the warp direction, whereas the carbon fibres ensure attenuation over a wide wave-band. The content of carbon fibres can range from 1 to 15 g/m2, with a preferred embodiment at around 5 g/m2. The diameter of the carbon fibres can range from 4 to 10 micron, with a preferred embodiment of around 7 micron and a specific electrical resistivity of around 18.106 ohm-meter, whereas the used fibre length can range from 0,4 to 8 mm, with a preferrence of around 3mm. According to a preferred embodiment of the invention a mix is used in which 50 % of the fibres have an average length of 3mm and 50% of the fibres have an average length of 0,5 mm. The overall content of carbon fibre is around 5g/m2. The carbon fibres are oriented in such way as to follow the warp direction. Due to a combination of metal in the weft of the fabric and carbon in the warp direction of the coating layers, one achieves radar attenuation in all directions of the fabric. Once the fibres are incorporated in the coating matrix, this coating matrix is foamed by means of a foaming device to a maximal homogeneity, and applied onto the fabric and then calendered. Using the preferred materials for the coating layer, said layer will have a density of around 190 g/m2. In order to increase the radar attenuation in the different directions, the coated fabric is, according to a preferred embodiment of the invention, punched with a so called leaf pattern, ie. cut through with specific geometric figures looking like leaves or stylised leaves, in such a manner that when applying a tension to the fabric, said fabric deploys into a three dimensinal structure which is comparable to the leaves of a tree. The incident radar waves are thus reflected around in all directions. When measuring the specular reflection by means of the so called monostatic setting, in which an absorbing window provides for a planar wave of a defined size to reach the test object, one observes that the camouflaging anti-radar nets according to the invention, executed with the characterizing features as described above, offer 20 to 100% better results than previous anti-radar nets with only metal fibres, or with a metal film or foil. The specular reflection is expressed in dB as the difference ( PI - P2 ) of the measured energy of the reflected signal of the camouflaging net ( P2 ) with respect to the energy reflected by a metal plate (PI) . For a wave frequency of 8 to 12 GHz one obtains, with the anti-radar nets according to the invention, average attenuations of 16,3 dB (± 1,9) in the vertical direction, and average attenuations of 12,2 dB (± 2,0) in the horizontal direction, as compared to average specular reflections of 7,8 dB (± 1,9) respectively 9,5 (± 1,6) with anti-radar nets of the previous type. With wave frequencies of 94 GHz one also obtains attenuations of above 10 dB.

Claims: C L A I M S . 1. Anti-radar nets consisting of a fabric and a coating layer on both sides of the fabric, characterised in that the weft threads of the fabric are provided with metal fibres and at least one of the coating layers is provided with carbon fibers. 2. Anti-radar nets according to claim 1, characterised in that the metal content of the net is in the range from 0,5 to 10 g/m2. 3. Anti-radar nets according to claim 1 or 2, characterised in that the metal fibres provided in the weft yarns have a diameter in the range from 4 to 10 micron and a length in the range from 30 to 50 mm. 4. Anti-radar nets according to any one of the preceding claims, characterised in that the carbon content of the net is in the range from 1 to 15 g/m2. 5. Anti-radar nets according to any one of the preceding claims, characterised in that the carbon fibres have a diametre in the range from 4 to 10 micron and a length in the range from 0,4 to 8mm. 6. Anti-radar nets according to any one of the preceding claims, characterised in that the carbon fibers on the fabric are oriented in the warp direction. 7. Anti-radar nets according to any one of the preceding claims characterised in that the fabric is punched in a leaf pattern. 8. Process for manufacturing anti-radar nets consisting of a fabric and a coating layer on both sides of the fabric, characterised in that the fabric is provided with a weft of which two shots comprise a mixed polyamide/rust resistant metal yarn, and one shot comprises a high tenacity polyamide. 9. Process for manufacturing anti-radar nets consisting of a fabric and a coating layer on both sides of the fabric, characterised in that the fabric is provided with the coating layers by using a foaming technique, in which carbon fibres are mixed and distributed into the coating matrix of at least one of the coating layers, and are then calendered. 10. Process for manufacturing anti-radar nets according to claim 8 or 9, characterised in that the obtained fabric is subsequently punched with a leaf pattern

Inventors: SMISSAERT LIEVEN, SMISSAERT, LIEVEN
Applicants: SEYNTEX N V, SEYNTEX N.V.
Application year: 1994
Main classification: F41H3
IPC codes: D03D15, F41H3, H01Q17